Water Pump loses prime when water tank level is 1/3 full

First, the gauge is not very reliable. Next, the pickup port (a threaded hole) on many trailers is well above the tank bottom, so the pump will suck air at the top edge of that hole. Do what Dutchman said and measure the remaining water, I'll bet it is quite a bit. Now to fix the problem, connect the supply hose to the drain port on the bottom with a tee and plug the original.